Thiocyanate-free cyclometalated ruthenium(II) sensitizers for DSSC: a combined experimental and theoretical investigation.

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Thiocyanate-free cyclometalated ruthenium(II) sensitizers for DSSC: A combined experimental and theoretical investigation† Ramesh Kumar Chitumalla,a Kankatala S. V. Gupta,a Chandrasekhram Malapaka,a Reza Fallahpour,c Ashraful Islam,b Liyuan Han,b Bhanuprakash Kotamarthi*a and Surya Prakash Singh*a In an effort to bring out efficient thiocyanate-free dyes for dye sensitized solar cells (DSSC) we have designed, synthesized and characterized four novel cyclometalated ruthenium(II) dyes (M1 to M4) with superior photochemical properties. All dyes contain terpyridyl ligands (TPY) with carboxylic acids as anchoring groups and cyclometalated ligand (TPY-C) with substituents for fine tuning the electronic properties. We obtain a broad absorption band which extends up to 725 nm due to metal to ligand charge transfer (MLCT) when donating groups are used, which slightly blue-shifts when a withdrawing group is used. In addition to the CT, small HOMO–LUMO gaps are obtained from electrochemical measurements which indicate characteristics of an ideal

Received 26th August 2013, Accepted 14th November 2013

sensitizer. All four dyes were used as sensitizers for DSSC and photoelectrochemical measurements were carried

DOI: 10.1039/c3cp53613k

these dyes adsorbed on the (TiO2)38 cluster. They revealed that, in bidentate bridging mode the dyes preferably bind with the help of two carboxylic groups onto the TiO2. To the best of our knowledge we are the first to do

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DFT studies of thiocyanate free cyclometalated ruthenium(II) dyes tethered to TiO2.

out. Reasonably good efficiency (7.1%) has been achieved for M3. We have carried out periodic-DFT studies of

1. Introduction Replacement of limited fossil fuels resources as a source of energy by solar energy which is abundant has been receiving considerable attention in recent times.1–3 While efficient solar cells based on the expensive silicon technology are available, a global challenge is to bring out alternative low cost solar cell technology for sustainable development.4 In the multitude of architectures and materials, thin film technology has received much attention due to its versatility and within the thin film technologies the DSSC which is based on nanocrystalline TiO2 is favored due to the potential low fabrication cost and reasonably high conversion efficiency.5–7 In DSSC photosensitization of the

a

Inorganic and Physical Chemistry Division, CSIR-Indian Institute of Chemical Technology, Hyderabad-500607, India. E-mail: [email protected], [email protected] b Photovoltaic Materials Unit, National Institute for Materials Science, 1-2-1 Sengen, Tsukuba, Ibaraki 305-0047, Japan c Institute of Organic Chemistry, University of Zurich UZH, Winterthurerstrasse 190, CH-8057 Zurich, Switzerland † Electronic supplementary information (ESI) available: Complete optimized geometrical parameters of the dyes M1–M4, selected FMOs, their energies and MO composition of the dyes M1, M2 and M4, TD-DFT data of the dyes M1, M2 and M4 on TiO2 with 1-COOH and 2-COOH and photoelectron spectrum of the dyes M1–M4@TiO2. This material is as ESI. See DOI: 10.1039/c3cp53613k

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TiO2 by the organic/organometallic dye is a crucial part of the conversion efficiency. Hence the design and synthesis of functional dyes have become a focus of current research in view of their potential applications as sensitizers in DSSC technologies.8 Molecules having a wide range of absorption in the visible region and containing an anchoring group like carboxylic or phosphonic acids are ideal candidates as sensitizers.9 The DSSC process requires that these dyes absorb sunlight and go to the excited state. From this excited state, an electron is then injected into the conduction band of TiO2 in a femtosecond lifetime through the anchoring group, and subsequently, in this process, the dye gets oxidized. The oxidized dye is then neutralized to the ground state by a redox system.10 The solar to power conversion efficiency of the DSSC is substantially affected by several factors such as, energy difference between the excited state dye and the conduction band of TiO2, grafting between the dye and the semiconductor (TiO2), and properties of the redox couple in the electrolyte.9 In addition, the electron density should be localized near the injecting group in the excited state.11 It has been shown that the redox potential of the dyes is pH independent in solution but developed a pH dependence upon adsorption on the TiO2 surface when the changes were investigated relative to the potentials of the semiconductor and the electrolyte solution.12 By choosing an appropriate sensitizer, tuning of the photoresponse of the semiconductor can be carried out.13

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Scheme 1

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Structures of the dyes M1–M4.

Ruthenium(II) polypyridyl complexes have been ideal choices as sensitizers due to attractive photophysical and photochemical properties.10,14 DSSCs employing ruthenium(II) polypyridyl complexes as sensitizers have achieved power conversion efficiencies of 11–12% under standard global air mass 1.5 (AM1.5G).10,15 The most successful charge transfer metal complexes employed so far in DSSC are cis-dithiocyanato bis-(2,2 0 -bipyridyl-4,4 0 dicarboxylato)ruthenium(II) (N3 or red dye) and trithiocyanato 4,4 0 ,400 -tricarboxy-2,2 0 ,6 0 ,200 -terpyridine ruthenium(II) (N749 or black dye), which yield overall conversion efficiencies of up to 10–11% under AM1.5G irradiation.16–18 Ruthenium(II) complexes equipped with thiocyanate (NCS) ligands have maintained a clear lead in performance among numerous dyes that have been scrutinized so far.1–10 Significant penetration of the DSSCs into the photovoltaic market over silicon based solar cells is hindered substantially by the longterm stability of the dyes and electrolytes under practical conditions.19–22 One of the reasons for this instability is the liberation of monodentate NCS ligands from the ruthenium centre.20,21 From basic coordination chemistry principles, it is a well known fact that the stability of a certain metal complex increases from the monodentate to polydentate ligand. So, many attempts to replace monodentate NCS donor ligands have been made.23–27 Replacement of the labile Ru–NCS bond with a cyclometalating ligand has been carried out in the recent times and reasonably high efficiencies have been obtained.28–32 The replacement of the NCS ligands with a cyclometalating ligand in ruthenium(II) complexes also gives the opportunity to independently tune both the metal-based highest occupied molecular orbital (HOMO) and the ligand p*-based lowest unoccupied molecular orbital (LUMO).33,34 Cyclometalated ruthenium(II) complexes of [Ru(N4C4N) (N4N4N)]+ configuration are a promising new class of molecular sensitizers for DSSCs as a result of their broad and red-shifted visible absorption in comparison to the analogous [Ru(N4N4N)2]2+ type coordinative complexes.28–32 In 2007, van Koten et al. documented a cyclometalated ruthenium(II) sensitizer in the form of a bis-tridentate complex [e.g., [RuII(tpy)-(pdcbpy)]+ (tpy = 2,2 0 :6 0 ,200 -terpyridine; pdcbpy = 4,4 0 -dicarboxy-6-phenyl-2,2 0 -bipyridine)].28 The power conversion efficiency of this dye was modest, but later in 2011, Berlinguette et al. with the derivatives of this complex, achieved


¨tzel et al., demonstrated that up to 8% efficiency.35 In 2009 Gra the cyclometalated complex [Ru(dcbpyH2)2(ppyF2)]+ [ppyF2 = 2-(2,4-difluorophenyl)pyridine] could generate power conversion efficiencies in excess of 10.1%36 and which was the first champion (i.e., Z 4 10) dye devoid of NCS ligands. In this paper as a part of our continuing effort to develop thiocyanate-free dyes24–26 we report the design, synthesis and characterization of four novel thiocyanate-free cyclometalated ruthenium(II) complexes M1, M2, M3 and M4 (Scheme 1) with superior photochemical properties. All four ruthenium(II) complexes have COOH as anchoring groups at 4,4 0 ,400 on the TPY ligand while the electronic properties are fine tuned by substituting electron withdrawing (M4) and donating groups (M2 and M3) on the bare TPY-C ligand (M1). To understand the behavior of the molecules M1–M4 both in the isolated form as well as in the adsorbed form on the surface of the TiO2, we make use of characterization techniques like UV-Vis, UV-diffuse reflectance spectroscopy (UV-DRS), PES and also computational chemistry methodologies namely the density functional theory (DFT) methods. The DSSC performance for all the four complexes has been investigated.

2. Experimental 2.1.

Materials and instruments

All reactions were carried out under nitrogen atmosphere (Scheme 2). Solvents were distilled from appropriate reagents. All reagents were purchased from Sigma-Aldrich. 1H NMR spectra were recorded on Avance 300 spectrometer. Chemical shifts were reported in parts per million (d) downfield from tetramethylsilane (TMS) as an internal standard in CDCl3. Low resolution mass spectrometry was performed using LCQ iontrap mass spectrometer (Thermo Fisher, Sanjose, CA, USA) equipped with an ESI source. IR spectra were recorded on a Perkin-Elmer 1800 series FTIR spectrometer and samples were analyzed as thin films on KBr pellets. 2.2. Fabrication of DSSC and photo-electrochemical measurements A double-layer TiO2 photoelectrode (14 + 5) mm in thickness with a 14 mm thick nanoporous layer and a 5 mm thick

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Scheme 2

Synthetic schemes, reagents and conditions.

scattering layer (area: 0.25 cm2) was prepared by screen printing on conducting glass substrate. A dye solution of 3 104 M concentration in acetonitrile/tert-butyl alcohol (1/1, v/v) was used to uptake the dye on to the TiO2 film. Deoxycholic acid (DCA) (20 mM) as a co-adsorbent was added into the dye solution to prevent aggregation of the dye molecules. The TiO2 films were immersed into the dye solution and then kept at 25 1C for 30 h. Photovoltaic measurements were performed in a sandwich type solar cell in conjunction with an electrolyte consisting of a solution of 0.6 M dimethylpropyl-imidazolium iodide (DMPII), 0.05 M I2, 0.1 M LiI and 0.5 M tert-butylpyridine (TBP) in acetonitrile (AN). The dye-deposited TiO2 film and a platinum-coated conducting glass were separated by a Surlyn spacer (40 mm thick) and sealed by heating the polymer frame. Photocurrent density–voltage (I–V) of sealed solar cells was measured under AM 1.5G simulated solar light at a light intensity of 100 mW cm2 with a metal mask of 0.25 cm2. The photovoltaic parameters, i.e. short circuit current ( Jsc), open circuit voltage (Voc), fill factor (FF), and power conversion efficiency (Z) were estimated from I–V characteristics under illumination. 2.3.

Synthesis and characterization

2.3.1. Synthesis of M1. In a typical experiment, Ru1 (424 mg, 0.6457 mmol), L-01 (150 mg, 0.6457 mmol) and 4-ethylmorpholine (0.5 ml) were dissolved in a mixture of methanol, water and THF (210 ml, 5 : 1 : 1) and the reaction mixture was stirred at reflux for 18 h under N2. Then the solvent was removed, the resulting residue was dissolved in a mixture of DMF, triethylamine and water (50 ml, 3 : 1 : 1) and again refluxed for 36 h. After that the reaction mixture was allowed to room temperature and the solvent was removed under reduced pressure. The resulting solid was filtered, washed with diethylether and finally purified on Sephadex LH-20 column using methanol and dichloromethane (1 : 1) as the eluent. The dark brown band was collected and evaporated to dryness to afford M1 as powdery material. 1 H NMR (CDCl3 + DMSO-d6, 300 MHz) d 8.74–8.64 (m, 4H), 8.09–8.04 (m, 3H), 7.91–7.78 (m, 7H), 7.64–7.57 (m, 2H), 7.33–7.27 (m, 3H). IR (KBr, cm1) 3417, 2923, 1599, 1536, 1399, 1363, 1273, 1234, 1152, 1017, 916, 779, 762, 696. Mass: m/z 698 (M + 1+).


2.3.2. Synthesis of M2. M2 was synthesized as described for M1 from Ru1 (400 mg, 0.6089 mmol), L-02 (150 mg, 0.6089 mmol) and was obtained as powdery material. 1 H NMR (CDCl3 + CD3OD, 300 MHz) d 9.39 (s, 1H), 8.99 (s, 3H), 8.07–8.05 (m, 4H), 7.76 (s, 1H), 7.59–7.52 (t, 2H), 7.43–7.41 (t, 2H, J = 7.648 Hz), 7.16–7.12 (d, 2H), 6.87–6.82 (t, 2H, J = 6.515 Hz), 2.59 (s, 3H). IR (KBr, cm1) 3406, 2974, 2936, 2738, 2677, 2491, 1709, 1603, 1469, 1357, 1234, 1117, 1035, 780, 618. MS (ESI) m/z: 712 (M + 1)+. 2.3.3. Synthesis of M3. M3 was synthesized as described for M1 from Ru1 (378 mg, 0.5762 mmol), L-03 (150 mg, 0.5762 mmol) and was obtained as powdery material. 1 H NMR (CDCl3 + CD3OD, 300 MHz) d 9.31 (s, 2H), 8.94 (s, 2H), 8.18–8.21 (d, 2H), 7.52–7.57 (m, 2H), 7.41–7.43 (d, 2H), 7.22–7.24 (d, 2H), 7.07 (s, 1H), 6.86–6.88 (d, 2H), 6.57–6.58 (m, 2H), 3.18–3.26 (m, 8H), 3.01 (s, 6H), 1.60–1.70 (m, 8H), 1.41–1.47 (m, 8H), 1.03–1.01 (m, 12H). IR (KBr, cm1) 3401, 3077, 2946, 2872, 2738, 2675, 2491, 1612, 1533, 1449, 1384, 1349, 1306, 1280, 1230, 1024, 785, 748, 732. Mass: m/z 726 (M + 1+). 2.3.4. Synthesis of M4. M4 was synthesized as described for M1 from Ru1 (356 mg, 0.5434 mmol), L-04 (150 mg, 0.5434 mmol) and was obtained as powdery material. IR (KBr, cm1) 3405, 2929, 2936, 2862, 2670, 2489, 1702, 1604, 1403, 1356, 1242, 1122, 1010, 786, 699. 2.4.

Computational details

DFT calculations have been performed using Gaussian 09 (G09) ab initio quantum chemical software package37 to fully optimize the closed shell configurations of the four ruthenium(II) complexes having a charge of +1. The theoretical singlet equilibrium structures were obtained when the maximum internal forces acting on all the atoms and the stress were less than 4.5 104 eV Å1 and 1.01 103 kbar respectively. The minima were further confirmed by vibrational analysis. No symmetry constraints were applied during the geometry optimizations. The gas phase relaxations of the atomic positions of all four ruthenium(II) complexes have been carried out by employing the hybrid Becke, three-parameter,38,39 Lee–Yang–Parr40 exchange– correlation functional (B3LYP) and a mixed basis set where 6-31G(d,p) basis functions were used on H, C, N, O, F and P atoms


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and a ‘‘double-z’’ quality basis set consisting of Hay and Wadt’s effective core potentials (LanL2DZ ECP)41–44 was employed on a ruthenium atom as implemented in G09. The relativistic ECP replaced the inner core electrons of the ruthenium metal atom, leaving the outer core (4s24p6) electrons and the 4d6 valence electrons to be concerned. A PF6 counter ion was added to the optimized geometries and then re-optimized at the same level of theory. The PF6 counter ion was placed at a distance ca. 6 Å from ruthenium centre towards TPY-C. The minimized geometries were then used to obtain the frontier molecular orbitals (FMOs) and also subjected to the single-point time-dependent DFT (TD-DFT) studies (first 60 vertical singlet–singlet transitions) to obtain the UV/VIS spectra of the dyes. The integral equation formalism polarizable continuum model (PCM)45,46 within the self-consistent reaction field (SCRF) theory, has been used for TD-DFT calculations to describe the solvation of the dyes in N,N-dimethylformamide (DMF) solvent. The TD-DFT calculations were also performed at B3LYP functional and the basis set as described above. Understanding the nature of binding and ground to excited state charge transfer that occurs from the adsorbed dye to the TiO2 surface requires modeling of the assemblies (dye@TiO2).47 Models of the dyes being adsorbed on the surface of TiO2 followed in the literature can be broadly divided into two types. Many reports deal with the periodic DFT calculations in which the TiO2 101 surface is created by cleaving the TiO2 anatase crystal and then carrying out the calculations using plane-wave DFT methods.48,49 Usage of the hybrid DFT methods estimates a reasonable band gap of the crystal which can be compared to experimental values.50,51 The major hurdle in handling the computational procedures for large molecules is the size of the vacuum slab which should be large to avoid the interactions of the dye with bottom of the slab above. The other method of modeling the dye@TiO2 is the creation of the nanoparticle (TiO2)n, where n is 16, 28, 38, 46, 60, 68 and 82.52–57 These nanoparticles have been created by appropriately ‘‘cutting’’ an anatase slab, exposing the majority (101) surface.55 The diameters of these nanoparticles vary from 0.6 nm to 2.0 nm. While the smaller nanoparticle due to the limited diameter has the disadvantage that all types of binding modes may not be possible to be studied, nevertheless the computational advantage in using this model has made this model very attractive and popular in the recent times.53,54,58,59 The calculations of the dye@TiO2 are carried out as isolated molecules mostly in gas phase but there are some reports of the calculations in solvent also.58,59 DFT with large local Gaussian/Slater functions are of choice for carrying out this study. One major advantage of this method is the interpretation of the nature of charge transfer in terms of orbitals by using the dye@TiO2 as an isolated molecule and carrying out the TD-DFT calculations.59 To understand the charge transfer process occurring from the adsorbed dye to the anatase TiO2 101 surface we have used the cluster models and performed the studies using Vienna Ab initio Simulation Package (VASP 4.6).60,61 We have chosen the (TiO2)38 neutral cluster models for our calculations in this work which strikes a balance between computational simplicity and


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accuracy. This cluster has been created from the anatase TiO2 by Persson et al.55 and TD-DFT studies have indicated the usefulness of this cluster with the dye adsorbed on it to understand the charge transfer process.54 But due to convergence problems with the recommended local Gaussian functions for the optimization of the (M1–M4)@TiO2 in G09 we had to use the plane-wave methods.58 Thus, unlike the earlier models used where in the full geometry optimization was carried out using localized functions, we have instead used a gamma centered grid and the periodic boundary conditions with a super-cell of 2.5 2.5 2.5 nm3 for this nanocluster (as an isolated molecule) and optimized it using plane waves and Projector Augmented Wave methods62,63 employing Generalized Gradient Approximation (GGA) of Perdew–Burke–Ernzerhof64 to describe the exchange and correlation effects. The Kohn–Sham orbitals are expanded in a plane-wave basis set with a kinetic energy cutoff of 400 eV. All the geometries were relaxed without any symmetry constraints until the force on each atom was less than 1 meV Å1. In the calculations of all charged systems, a uniform background charge was introduced to keep the system charge neutral within the supercell. The spurious electrostatic interactions due to the introduction of the uniform background charge, and that associated with the long-range interaction between one supercell to the other, were corrected with monopole and multipole terms as implemented in VASP.65,66 The ground state geometries of the clean (TiO2)38 cluster and each dye are optimized first, and then, each ground state geometry of the dye@(TiO2)38 is fully optimized in VASP. Finally the optimized structures of dye@(TiO2)38 are used to calculate the fifty lowest singlet electronic transitions using TD-DFT method with G09 in DMF solution. The TD-DFT calculations with counter ion have been carried out at a lower basis set (B3LYP/3-21G*) which has been found to be suitable for this type of assemblies.50,58

3. Results and discussion 3.1.

Photoelectrochemical properties

Photocurrent action spectra of the dye-sensitized solar cells are shown in Fig. 1, where the monochromator was incremented through the visible spectrum to generate the IPCE (l) curve as defined below. Jsc IPCEðlÞ ¼ 1240 lF where l is the wavelength (nm), Jsc is the photocurrent density under short-circuit conditions (mA cm2), and F is the incident radiative flux (mW cm2). The incident photon-to-current conversion efficiency (IPCE) is dependent on the anchoring group. IPCE’s for all of these dyes are in the range 46–74% (Table 1). Among all the dyes the M3 dye has larger IPCE and M1 has smaller IPCE. The order of IPCE values for the four dyes is M3 4 M2 4 M4 4 M1. In the same table, the photovoltaic performance for M1–M4 is also depicted. The overall conversion efficiency (Z) of the DSSC is calculated from the integral photo-current density ( Jsc), the open-circuit photovoltage (Voc),


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Fig. 1

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IPCE spectra of the DSSCs sensitized with M1–M4 dyes.

Fig. 2 Current–voltage characteristics of the dyes M1–M4 under illumination intensity of 100 mW cm2.

Table 1 Short-circuit photocurrent density (Jsc), open-circuit photovoltage (Voc), fill factor (FF), overall conversion efficiency (Z), IPCE, of the dyes M1–M4

Jsc (mA cm2) Voc (V) FF Z (%) IPCE (%)

M1

M2

M3

M4

8.22 0.550 0.69 3.1 46

10.85 0.550 0.72 4.3 55

16.78 0.615 0.69 7.1 74

9.31 0.570 0.74 3.9 48

the fill factor of the cell (FF) and the intensity of the incident light (Iph), using the equation given below, Z ¼ Jsc Voc

FF Iph

The FF is defined by the following equation, FF ¼

JphðmaxÞ VphðmaxÞ Jsc Voc

where Jph(max) and Vph(max) are the photocurrent and photo voltage for maximum power output. The current density–voltage (I–V) curves measured under illumination of 100 mW cm2 and AM1.5G solar light for a series of cells prepared with the dyes M1–M4 are depicted in Fig. 2, while the corresponding short-circuit current density ( Jsc), fill factor (FF) and Voc parameters are presented in Table 1. The Jsc value, upon substitution on TPY-C ligand has been increased substantially from M1 (8.22 mA cm2) to the remaining dyes. In the case of M3 (16.78 mA cm2) it has been observed that the increment is more than two fold, which makes M3 more efficient among the other dyes. The Voc value for the dyes M1 (0.550 V) and M2 are the same but increased for the dyes M3 (0.615 V) and M4 (0.570 V). The FF for the dyes M1 and M3 are found to be same (0.69) and for the dyes M2 and M4 are 0.72 and 0.74 respectively.


3.2.

Geometries and orbitals of the isolated ruthenium dyes

To obtain the conformations and the geometries of the isolated dye molecules M1–M4 (Scheme 1), we have carried out geometry optimization making use of the computational methods described above. It is well known from literature that the DFT methodologies predict the geometries of the ruthenium(II) dyes which are comparable to the experimental determined geometries.67 Of these methods the functional B3LYP with a LANL2DZ basis set on ruthenium and 6-31G(d,p) basis set on the other atoms strikes a balance between accuracy and the computer time and hence has been used here.68 This combination has also been used with success on ruthenium dyes in our earlier studies.69 The crystal structure of a cyclometalated ruthenium(II) complex from the literature (CSD) with different substitutions, which is closest to the dyes studied here was used as the starting structure for the geometry optimization.30 In this starting structure the binding ligands adopt a flat geometry and are mutually perpendicularly coordinated to the metal centre. Due to the ruthenium(II) oxidation state the dyes have +1 charge on them as shown in Scheme 1 which is neutralized by counter ion in solution in the experiment. To simulate this where necessary in computational studies we have used a counter ion which is taken to be PF6. The geometrical parameters obtained by optimizing the molecule in the presence of a counter ion are shown in Table 2. Here various positions of the counter ion have been tried out and the final position obtained is in good agreement with the reported crystal structures of some ruthenium(II) dyes with counter ions.30,70 For clarity reasons we show only the metal–ligand geometrical parameters in the table, the other geometrical parameters are given in ESI.† It is observed that the overall structures of all the four dyes are similar with the two ligands TPY and TPY-C almost perpendicular to each other. It is clearly seen from Table 2 that the Ru–C bond distances are in the range 1.980–1.995 Å as predicted by B3LYP level of theory for optimization. The variation with respect to the substituted groups is brought out clearly with the shortening


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Table 2

M1 M2 M3 M4

Optimized geometrical parameters of the dyes M1–M4 in the most stable conformation (bond lengths in Å and bond angles in deg)

Ru1–C3

Ru1–N6

Ru1–N2

Ru1–N4

Ru1–N5

Ru1–N7

N2–Ru1–C3

C3–Ru1–N4

N5–Ru1–N6

N6–Ru1–N7

1.987 1.987 1.995 1.980

2.053 2.052 2.052 2.057

2.152 2.153 2.127 2.155

2.141 2.143 2.116 2.140

2.121 2.120 2.125 2.123

2.103 2.103 2.101 2.106

77.6 77.4 78.2 77.8

77.7 77.6 78.4 77.8

77.8 77.9 77.8 77.7

77.7 77.7 77.8 77.7

of this bond when an electron withdrawing group is substituted (M4). It is almost 0.015 Å shorter than the Ru–C bond as compared to that of the M3 dye. In the case of electron donating substituents on the TPY-C (M2 and M3) only when the donating groups are two methyls (M3) we notice slight elongation of the bond length of 0.008 Å. The other important bond lengths are that of Ru–N. Here the Ru–N bond lengths of the TPY-C are larger than the Ru–N bond lengths seen in TPY ligand (almost a difference of 0.030 Å). Since we do not have the experimental data of these molecules we compare the theoretically obtained bond lengths with that of the experimental bond lengths of some cyclometalated ruthenium complexes in the literature.30,71 We find they are within the same range of the predicted values shown in Table 2. FMOs are very important in this context to understand the nature of charge transfer, within the molecule and from the molecule to the semiconductor surface, when transition occurs from the ground to the excited state. An accurate description of the orbital energies is also necessary to get an insight into the charge transition. The change in orbital energies is expected when solvent effects are taken into account for molecules in general.72 But here additionally the orbital energies are expected to change also when the counter ion is included into the calculation.70 The energy levels are shown in Table 3 for M3 while for the other molecules they are shown in the ESI.† The HLG for M3 is 2.47 eV while the HOMO at 5.20 eV and LUMO at 2.73 eV. In the case of M2 the HLG is around 2.42 eV. When the substituted group is an electron withdrawing group like the case of M4, we notice that the HLG is by 2.64 eV larger than what was obtained for the other molecules. The characters of the orbitals have been determined and are given in the same table for M3 only while for the other dyes are shown in ESI.† The HOMO for M3 dye consists of B48% ruthenium and B38% TPY-C. The LUMO consists of B13% ruthenium and B82% TPY ligand. It is also interesting to see that all four dye molecules have an almost similar orbital character in spite of the change in the substituents on TPY-C which range from donor to acceptors. 3.3.

UV-Vis absorption properties

The UV-Vis absorption spectra of the four sensitizers in DMF solvent are depicted in Fig. 3, and the corresponding numerical data are summarized in Table 4. In addition to the higher energy p–p* absorptions the dyes also exhibit MLCT absorptions. The absorption wavelength corresponding to the main MLCT has been decreased gradually from M1 (515 nm) to M3 (500 nm) through M2 (504 nm) as the electron donating methyl group(s) are increased on TPY-C, whereas the M1 dye does not have any substitution on its


Table 3 Selected FMOs (isosurface = 0.02), their energies (eV) and molecular orbital composition (%) of the dye M3. (For M1, M2 and M4 are given in the ESI)

FMOa

MO compositionb (%)

LUMO + 2

TPY (97.4)

LUMO + 1

Ru (5.1) TPY (93.9)

LUMO

Ru (12.6) TPY (81.6) TPY-C (5.7)

HOMO

Ru (47.5) TPY (14.2) TPY-C (38.2)

HOMO 1

Ru (56.3) TPY (20.3) TPY-C (23.3)

HOMO 2

Ru (60.7)TPY (13.0) TPY-C (26.2)

a

FMOs obtained from GaussView-4.1 (ref. 77). b Compositions greater than 5% are tabulated and are obtained from VMOdes program (ref. 78).

TPY-C. Hence a hypsochromic shift was observed from M1 to M3. On the other hand, contrary to this, in the case of M4 dye, the absorption wavelength corresponding to main MLCT was increased from 515 nm (M1) to 535 nm (M4) as expected, due to the presence of an electron withdrawing carboxylic group on its TPY-C. A bathochromic shift was observed from M1 to M4, hence M4 has the maximum MLCT absorption wavelength among four dyes.


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Optical absorption spectra of the dyes M1–M4 in DMF solution.

Fig. 3

follow similar trends in intensities as explained in the case of isolated dyes. The dyes M1, M2 and M3 show almost similar absorption properties for one p–p* band and two MLCT bands in which the dye M3 has red shifted absorption from that of M1 and M2. The dye M3 has more intense absorption bands compared to that of other dyes which makes it the most efficient in DSSC performance. Due to the presence of two electron releasing methyl substitutions on its TPY-C ligand the dye M3 showed red shifted and intense absorption bands. The high intense p–p* band has been observed at ca. 409 nm and the two MLCT bands were observed at ca. 490 nm and at ca. 563 nm. In the case of M4, the p–p* and MLCT bands were observed at 469 (however with a small intensity) and 549 nm respectively. The solid state optical absorption spectra of the dyes M1–M4 adsorbed on TiO2 thin film is given in Fig. 4 and the corresponding data in Table 5. 3.4.2. Photoemission yield spectrometer (AC-3E) and E0–0 measurements. The ionization potential values (IP) of the dyes M1 to M4 bound to nanocrystalline TiO2 film have been

The absorption wavelengths of the four dyes corresponding to p–p* did follow the trend as explained above and the values were ca. 380 nm. The molar extinction coefficients (e) corresponding to MLCT bands were increased from M1 to M3 and decreased for M4 i.e., the dye M3 has broader and more intense envelope relative to the other three dyes. TD-DFT calculations are employed to understand the nature of the excited states and examine the vertical excitation energies of the four ruthenium(II) dyes (Table 4). The most intense singlet transition in the low-energy region for the dye M3 is contributed predominantly by HOMO 1 - LUMO + 1 ( f = 0.0909 and 527 nm). This can be classified as MLCT as the HOMO 1 is composed of Ru (56.3%) + TPY (20.3%) + TPY-C (23.3%), whereas LUMO + 1 is composed of mainly TPY (93.9%). The other two transitions shown in the same table for M3 are of lesser intensity and these occur at 565 nm ( f = 0.0486) and at 636 nm ( f = 0.0257). 3.4.

Adsorption of dyes on the TiO2

3.4.1. Solid state absorption of dyes on mesoporous TiO2. The absorption spectra of the four dyes tethered to TiO2 also Table 4

Fig. 4 Solid state optical absorption spectra of the dyes M1–M4 adsorbed on 12 mm TiO2 thin film.

Experimental absorption maxima (lexp), calculated absorption (lcal), oscillator strength (f) and excitation energies (Eexc) of the dyes M1–M4

lExp (nm)

lCal (nm)

State

f

EExc (eV)

Dominant contributiona

M1

515

623 559 521

S2 S4 S5

0.0250 0.0552 0.0926

1.99 2.22 2.38

H 2 - L (88%):MLCT H 1 - L (66%):MLCT & H 2 - L + 1 (25%):MLCT H 1 - L + 1 (91%):MLCT

M2

504

627 561 523

S2 S4 S5

0.0244 0.0580 0.0922

1.98 2.21 2.37


M3

501 545

636 565 527

S2 S4 S5

0.0257 0.0486 0.0909

1.95 2.19 2.35


M4

535

607 546 510

S2 S4 S5

0.0257 0.0579 0.0898

2.04 2.27 2.43


a

Values obtained using the program GaussSum-2.1.6. (ref. 79).



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PCCP

Table 5 Experimental photophysical (UV-DRS) and electrochemical properties of the dyes M1 to M4@TiO2

l On TiO2 (nm)

Absorbance (au)

HOMO (V)

E0–0 (eV)

LUMO (V)

M1

410 490 559

0.48114 0.38100 0.32350

5.22

1.79

3.43

M2

408 491 563

0.57155 0.43703 0.35776

5.57

1.81

3.76

M3

409 491 566

0.70471 0.54087 0.45785

5.58

1.84

3.74

M4

469 549

0.32552 0.27378

5.46

1.78

3.68


Dye@TiO2

determined using the photoemission yield spectrometer (Riken Keiki, AC-3E). The ground state oxidation potential values for the dyes M1, M2, M3 and M4 in eV are 5.22, 5.57, 5.58 and 5.46 respectively and these values are sufficiently low as compared to the redox potential of I/I3 (5.20 eV)73 for efficient regeneration of oxidized dye through the reaction with iodide. The HOMO energy levels are negatively shifted upon substitution on TPY-C from M1 to remaining dyes. The shift of HOMO from M1 to M2 and M3 is almost same and is ca. 0.35 eV whereas the shift of HOMO from M1 to M4 is only 0.24 eV. The difference in the shift of HOMO may be attributed to the nature of substitutions, i.e., in the earlier case the substitutions have an electron donating nature and in the latter case the substitution has an electron withdrawing nature. The onset of the optical energy gap (E0–0) of the dyes M1, M2, M3 and M4 in eV are 1.79, 1.81, 1.84 and 1.78 respectively. The trend in E0–0 i.e., increase from M1 to M3 and decrease for M4 is again due to the same reason as explained above (vide supra). The excited state oxidation potentials of the dyes M1 to M4 in eV are 3.43, 3.76, 3.74 and 3.68 respectively, which lie well above

Scheme 3

the conduction band edge (4.2 eV)74 of the nanocrystalline TiO2. The excited state oxidation potential is obtained by subtracting the E0–0 value from its corresponding ground state oxidation potential. The complete details are summarized in Table 5. All corresponding photoelectron spectra are given in the ESI.† 3.4.3. Computational studies of adsorption configurations. To understand in greater detail the nature of adsorption of the dye on TiO2 surface we have carried out modeling of the dye@TiO2 as explained in the computational details. As there are a multitude of adsorption configurations like monodentate ester (ME), bidentate chelating (BC) and bidentate bridging (BB) etc., particularly for organometallic systems we have in this study only concentrated on the BB configuration.49,75,76 This we feel is sufficient to understand the actual charge transfer process taking place between the dye and TiO2 surface in this class of dyes. The possible modes of binding the dye (here we take only those of M3) are shown in Scheme 3. We have considered only the dissociative BB mode as this type of adsorption is known to be mostly favored.76 In Scheme 3 the binding of 1-COOH (a) and 2-COOH (b) groups is shown. The dyes are adsorbed on (TiO2)38 in dissociative BB configuration with 1-COOH and 2-COOH groups and then optimized with VASP. The optimized distances between oxygens of carboxylic group (Oc) and Ti are as follows, in the case of 1-COOH adsorption, these distances are 2.08 and 2.18 Å and in the case of 2-COOH the distances are 2.03, 2.11 and 2.14, 2.17. We found that the 2-COOH adsorption is energetically more favored than that of 1-COOH by 14 kcal mol1. For 2-COOH adsorption these distances are smaller as compared to that of 1-COOH adsorption, which indicates strong binding. Important FMOs obtained for M3@TiO2 are shown in Table 6. In the 2-COOH case, it is clear that the HOMO 2 and HOMO are mainly localized on the dye and the LUMO, LUMO + 5, LUMO + 6, LUMO + 12 and LUMO + 16 are localized on TiO2. Similarly for 1-COOH case the HOMO 2, HOMO 1 and HOMO are mainly localized on the dye and the LUMO, LUMO + 9, LUMO + 10 are localized on TiO2. FMOs for the

Possible adsorption configurations of the dye M3 on (TiO2)38.



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Table 6 Selected FMOs and their energies (eV) of 2-COOH and 1-COOH adsorption of PF6–M3 on (TiO2)38

Table 6

(continued)

2-COOH

1-COOH

1-COOH


2-COOH

remaining dyes@TiO2 are shown in ESI.† TD-DFT studies of the dye@TiO2 are carried out and the results for M3@TiO2 are shown in Table 7. From the table it is clear that, in 2-COOH case CT occurs from HOMO 1 (from dye) to LUMO + 7 (TiO2) and LUMO + 9 (TiO2) at 676 nm and the most intense peak in this case is observed at 567 nm which corresponds to mixed transitions of HOMO 2 to LUMO + 7, LUMO + 21 and LUMO + 25 (a CT from the dye to TiO2). Similarly in 1-COOH case CT occurs from HOMO 1 (from dye) to LUMO + 11 (TiO2) and LUMO + 18 (TiO2) at 658 nm and the most intense peak in this case also is observed at 567 nm which corresponds to HOMO 2 to LUMO + 11 and LUMO + 18 (a CT from the dye to TiO2). TD-DFT results of the remaining dyes@TiO2 are given in the ESI.†

4. Conclusions Cyclometalation of the metal is shown to be an effective way to increase the electron density at the metal center, which raises the energy of the HOMO. The photophysical and electrochemical properties of these complexes can be further tuned using suitable electron-withdrawing and electron-donating groups on TPY-C ligand. The sensitizer M3 shows distinctive red-shifted MLCT absorption which improves the light harvesting ability in the visible and NIR regions, resulting in the best overall conversion efficiency (Z = 7.1%) among the four sensitizers studied, which is 70% as compared to that of the benchmark N749 (10.1%). From the dye M1 to M3 the electron donating groups are increased which results in the bigger electron injection rate that causes enhanced PCE for M3. The efficiency of the dye M4 (3.9) is by 26% greater as compared to that of M1 (3.1%), due to the presence of one extra COOH group in M4, through which more charge can be injected into the TiO2.



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Table 7

PCCP TD-DFT data of M3@(TiO2)38 with 2-COOH and 1-COOH adsorption

Dye adsorbed with

State

Wavelength (nm)

Excitation energy (eV)

Oscillator strength

Dominant contribution

2-COOH

S7

676

1.83

0.0302

S14 S26

605 567

2.05 2.19

0.0061 0.1188

S27

564

2.20

0.0048

H 1 - L + 7 (25%), H 1 - L + 9 (14%) & H 1 - L + 8 (12%) HOMO - L + 5 (24%) & HOMO - L + 6 (17%) H 2 - L + 7 (11%), H 2 - L + 21 (11%), H 2 - L + 25 (11%), & H 2 - L + 28 (18%) HOMO - L + 12 (15%) & HOMO - L + 16 (24%)

S8 S22 S43 S45

658 567 515 513

1.89 2.19 2.41 2.42

0.0160 0.0741 0.0225 0.0145

H H H H

1-COOH

Detailed DFT/TD-DFT calculations were carried out for the four dyes and also for dye@TiO2 assemblies. The TD-DFT results suggest that the dyes exhibit direct charge transfer to TiO2 due to excitation. DFT calculations emphasize the importance of the presence of counter ion for isolated dyes as well for dye@TiO2 assemblies. The electronic absorption data of the dyes and dyes@TiO2 from UV and UV-DRS respectively, are compared with the TD-DFT data of dyes and dyes@TiO2 assemblies and are in very good agreement. The calculated electronic structure shows an alignment of the molecular orbitals of the dyes with the band structure of the TiO2.

Acknowledgements The authors thank the Director, CSIR-IICT for constant encouragement. R.K.C.H. thanks CSIR and K.S.V.G. thanks UGC New Delhi for SRF fellowship. S.P.S. thanks the DST for Fast Track Young Scientist Project (CS-83/2012).

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